This invention relates to hardpoints in superplastically formed multisheet sandwich structures, and more particularly to integral hardpoints joined to and joining the top and bottom face sheets of a superplastically formed, diffusion bonded metal sandwich structure by which the sandwich structure can be attached to adjacent structures, pivots, actuators and the like.
Multisheet expanded metal sandwich structures made by superplastic forming (SPF) and diffusion bonding have been in use for many years, particularly in the aerospace industry, because of their low cost, good strength and stiffness per unit weight, and high temperature resistance. Various processes for fabricating these structures have been developed in the past, with various degrees of success, and recently these processes have been improved to enable fabrication of these metal sandwich structures with exceptional quality, reliability and efficiency. The new processes are described in Provisional Patent Application No. 60/010,033 entitled xe2x80x9cMultisheet Sandwich Structuresxe2x80x9d, filed on Jan. 12, 1996 by Fred Buldhaupt, Dave Gane, Matt Kistner and Jeff Will.
One persistent problem that has been encountered with the use of these metal sandwich structures is connecting them with adequate load transfer capacity to adjacent structure in the assembly of which they are a part. The top and bottom face sheets of the sandwich structure are typically thin titanium sheets which cannot bear the application of concentrated forces in a single point, as would happen if the part were attached by sheet metal screws or bolts or the like. Moreover, the hole that such a fastener would make in the sheet metal would further weaken the sheet metal and facilitate the initiation of cracks in a vibrating or other fatigue environment.
The use of bolts or other fasteners of that type often require the fastener to be tightened into a state of tension to attain a secure, full strength connection. The use. of such fasteners with conventional metal sandwich structures is difficult because the necessary tensile preload exerts a crushing force on the sandwich structure which tends to collapse it, damaging the part and losing the necessary fastener preload.
Important applications of metal sandwich structures are in aerodynamic structures in aircraft, missiles and the like. For purposes of aerodynamic efficiency and/or reduced radar cross-section, fastener heads for such structures are often required to be recessed into a counterbore or countersink to lie flush with the surface of the part. However, such recessing of fastener heads is difficult or impossible with conventional fastening techniques for metal sandwich structures.
The xe2x80x9ccloseoutxe2x80x9d of a metal sandwich structure is the peripheral edge portion that spans the top and bottom face sheets. It provides an edge seal for the structure and often would be the ideal location for fastening to adjacent structure. However, existing closeout techniques may not provide a structure of the desired shape and strength to serve as a flush edge fastening structure, and may not provide an edge shape with a configuration that matches the configuration of the adjacent structure.
Thus, there has long been an unsatisfied need in the industry for a metal sandwich structure having integral hardpoints that enable the metal sandwich structure to be securely fastened to adjacent structure in a manner that distributes the connection forces evenly to the metal sandwich structure without damage, and providing the possibility of sinking the fastener heads flush with the outside surface of the part.
Accordingly, it is an object of this invention to provide a method of attaching a superplastically formed, diffusion bonded sandwich structure to adjacent structure. Another object of this invention is to provide a method of making a metal sandwich structure having an integral hardpoint at any desired locations in the structure, by which the sandwich structure can be connected to adjacent structures in the assembly in a manner that evenly distributes forces exerted by the adjacent structure on the sandwich structure. A further object of this invention is to provide a sandwich structure having an integral hardpoint by which the sandwich structure can be connected to adjacent structure in the assembly providing highload transfer capability and fatigue tolerance.
These and other objects of the invention are attained in a superplastically formed, diffusion bonded sandwich structure having an integral metal hardpoint, made by joining at least two superplastic metal sheets together into a pack of three or more sheets by welding or diffusion bonding the two sheets along a pattern of lines which form junction lines between the sheets when the pack is inflated by gas pressure at superplastic temperatures. At least one metal insert is interposed between at least two of the sheets and all of the sheets in the pack are sealed together around an outside peripheral edge to create a gas tight envelope. The pack is heated to superplastic temperatures and the top and bottom face sheets are diffusion bonded to top and bottom surfaces of the metal insert by application of heat and pressure from top and bottom inner surfaces of the die cavity. While at superplastic temperatures, the pack is inflated by gas pressure against inside surfaces of a die to form an expanded metal sandwich structure having integral webs and an integral hardpoint formed by the metal insert. After forming, the gas pressure is vented, the part is cooled below superplastic temperature and is removed from the die.